Current hormonal ablation therapy, the mainstay treatment for advance prostate cancer, is palliative, due the development of androgen-independent growth. Androgen receptor (AR) is expressed in most prostate cancer cells and AR overexpression of AR is sufficient and necessary for androgen-independent growth, which provides strong rationale for developing novel therapies against advanced prostate cancer through downregulation of AR. Dr. Dong's laboratory has identified a novel synthetic compound 6-amino-2-[2-(4-tert- butyl-pnenoxy)-ethylsulfanyl]-1H-pyrimidin-4-one, named DL3 for simplicity, as a potent AR antagonist. DL3 inhibits dihydrotestosterone (DHT)-stimulated cell growth and gene expression in all prostate cancer cell lines tested, including androgen-independent cells and cells resistant to antiandrogens, flutamide (Flut) or nilutamide (Nilut). The inhibitory effects of DL3 are more potent than bicalutamide (Bical), Flut, and Nilut. It inhibits neither AR nuclear localization nor DHT-induced AR NH2-terminus and COOH-terminus interaction and has no detectable AR agonist activity. DL3 reduces AR stability and downregulates of AR protein expression. It competes with DHT but not estradiol for the binding to cells. Docking analysis using protein crystal structure of AR ligand-binding domain (LBD) implies that DL3 can bind to the LBD. These observations prompted Dr. Dong to hypothesize that DL3 is a novel AR antagonist that binds to AR and induces the formation of inactive AR transcription machinery and AR degradation, and, hence, interrupts AR signaling. Three specific aims are proposed to test the hypothesis and to investigate efficacy of DL3 therapy against human prostate cancer cells in animals. In the specific aim 1, he will characterize the binding of DL3 to AR and identify amino acid residues of the DL3 binding site. He will determine the biochemical properties of the binding, investigate whether DL3 competes with DHT for AR binding, and identify amino acid residues of AR that interact with DL3. In the specific aim 2, he will investigate effects of DL3 on proteosome-mediated degradation of AR and assembly of AR transcription complex. He will determine effects of DL3 and Bical on AR stability, ubiquitination, and association with E3 ubiquitin ligase. By using the chromatin immunoprecipitation (ChIP) assay, he will investigate whether treatment with DL3 and Bical induce formation of transcription-inactive AR complex at the promoter region of prostate-specific antigen gene and identify cofactors in the complex. In the specific aim 3, Dr. Dong propose to investigate therapeutic effects of DL3 against human prostate cancer cells in mice. He will determine and compare effects of DL3 and Bical, alone or in combination with castration, on growth of tumors formed by both androgen-dependent and -independent cells and by cells refractory to Flut. He will determine DL3 distribution in tumor-bearing mice and correlate therapeutic effects with expression of AR and AR-target genes in tumors. These studies will firmly establish that DL3 is a novel AR antagonist and will enrich the understanding of mechanisms by which Bical interrupts AR signaling. Project Narrative Our preliminary studies have identified a novel synthetic compound 6-amino-2-[2-(4-tert- butyl-pnenoxy)-ethylsulfanyl]-1H-pyrimidin-4-one, named DL3 for simplicity. DL3 competitively inhibits the binding of DHT to human prostate cancer cells, but not estradiol to human breast cancer cells. It downregulates androgen receptor expression and suppresses androgen-induced cell growth and gene expression in prostate cancer cells. The proposed research will establish that DL3 is a novel AR antagonist, will elucidate mechanisms by which DL3 interrupts AR signaling, and will determine therapeutic effects of DL3 against human prostate cancer cells in mice. PUBLIC HEALTH RELEVANCE: Our preliminary studies have identified a novel synthetic compound 6-amino-2-[2-(4-tertbutyl-pnenoxy)-ethylsulfanyl]-1H-pyrimidin-4-one, named DL3 for simplicity. DL3 competitively inhibits the binding of DHT to human prostate cancer cells, but not estradiol to human breast cancer cells. It downregulates androgen receptor expression and suppresses androgen-induced cell growth and gene expression in prostate cancer cells. The proposed research will establish that DL3 is a novel AR antagonist, will elucidate mechanisms by which DL3 interrupts AR signaling, and will determine therapeutic effects of DL3 against human prostate cancer cells in mice.